The technique of using a plasma to provide energy to atoms, ions and/or molecules in a sample introduced thereto, which atoms, ions and/or molecules interact with the energy of the plasma and become "excited" thereby, is well known. The electrons of the atoms, ions and/or molecules of a sample which interact with the energy of a plasma are caused to move from lower energy state orbitals into higher energy state orbitals of their respective atoms, ions and/or molecules. When the electrons later return to their more stable lower energy state orbitals an electromagnetic (hereinafter EM), wave radiation, is caused to be emitted. The frequency spectrum of the emitted EM wave radiation is an identifying "fingerprint" of the atoms, ions and/or molecules present in the sample. In addition, interaction of sample with the energy present in a plasma can cause sample fragments, including both positive and negative ions, to be formed. Said sample fragments can be analyzed in mass spectrometry analysis systems to provide a sample identifying mass spectrum.
To introduce a sample to a plasma it is necessary to provide a means for producing a plasma at an intended and contained location. Said location must be situated and supported so that a sample to be analyzed can be easily introduced thereto, and so that EM wave analysis equipment or a sample fragment detector can monitor the resulting emitted EM wave radiation spectra or intercept sample fragments, respectively.
The relationship between the plasma location, sample introduction and EM wave or sample fragment intercepting analysis equipments, is typically established and maintained by a "torch". Torches are typically constructed from a series of concentric quartz tubes and ports which provide access to spaces formed between outer and inner surfaces of the various concentric tubes. It is possible to use other materials, such as ceramics etc. in constructing a torch as well, hence the term "tube" will be used independently in the following.
The innermost concentric tube is typically termed a sample injector tube based upon the function with which it is associated. In standardly available torches the sample injector tube is typically of a decreasing inner diameter as the longitudinal dimension thereof is transcended from the lower aspect thereof to the upper aspect thereof as viewed in side elevation, as better described below in conjunction with other elements of a standard torch. The purpose of the decreasing inner diameter is to cause a flow of sample to increase in velocity as it transverses the sample injector tube and exits therefrom at the upper aspect thereof prior to entering a plasma, again as better described below. Continuing, the sample injector tube is typically concentrically surrounded by a second, larger inner diameter tube, typically termed an intermediate tube. The upper aspect of the intermediate tube typically, but not necessarily, extends vertically past the upper aspect of the sample injector tube, as the combination of elements is viewed in side elevation from a distance perpendicularly removed therefrom, with the longitudinal dimensions of the tubes projecting upward, perpendicularly from an underlying horizontal surface. Surrounding the concentric combination of sample injector and intermediate tubes just described, is typically a third concentric tube of an inner diameter large enough to contain the identified combination of tubes. Said larger inner diameter tube is typically termed, appropriately, the outer tube. Viewed again in side elevation and in combination with the sample injector and intermediate tubes as described above, the upper aspect of the outer tube extends to a vertical level above the more centrally located tubes. It is within the space within the outer tube, but above the upper aspects of the more centrally located tubes that a plasma is typically caused to be formed during use of the torch to analyze a sample. This is normally effected by placing an electrically conductive coil around the outer surface of the outer tube, and energizing it with an electrical current at 27 Megahertz, or at some other plasma forming frequency. The plasma formed by this technique is typically termed an "Inductively Coupled Plasma", or "ICP" for short.
At the lower aspect, viewed as described above, of the torch concentric tube combination comprising the sample injector tube, intermediate tube and outer tube there are typically ports. One such port, the intermediate port, provides access to the annular space formed between the outer surface of the sample injector tube and the inner surface of the intermediate tube; and another such access port, the outer port, provides access to the annular space formed between the outer surface of the intermediate tube and the inner surface of the outer tube. Another port, the sample flow port, provides access to the space within the sample injector tube. In use a sample is introduced to the sample injector tube via the sample flow port and is forced by a driving pressure to transverse the length of the sample injector tube vertically and eject therefrom at the upper aspect thereof into the space in which a plasma can be formed, as described above. A gas is injected into each of the outer and intermediate ports in such a manner as to cause it to flow, typically, tangentially around and upward in the space within the torch associated with each said access port. Said gas flows can be described as following a spiral and upward locus. It is also possible to inject gas into one or both of the outer and intermediate ports so that it flows vertically, in a laminar manner, rather than tangentially in the torch. The basic purpose of said tangential or vertical gas flows is to insulate the various tubes which they contact, which tubes typically melt at approximately twelve hundred (1200) degrees centigrade or below, from the heat of a plasma, the temperature of which can exceed five thousand (5,000) degrees centigrade. Another purpose of the tangentially or vertically injected gas flows is to aid with positioning the plasma in the upper aspect of the torch. As well, the gas flow introduced into the intermediate port is especially useful in helping to initially create a plasma, and the gas flow injected into the outer port is especially useful in shielding the inner surface of the outer tube in the vertically upper aspect, plasma associated region thereof, from the heat of the plasma.
It is also important, as regards the present invention, to understand how a sample is prepared for analysis. Typically a pneumatic nebulizer is utilized. Said pneumatic nebulizer accepts a sample solution, and ejects said sample solution via a spray aperture, under pressure, to form nebulized sample solution droplets. A carrier gas flow might also be introduced to help cleave the ejected solution. A spectrum of various diameter droplets is formed by said action. The larger diameter droplets, (e.g. larger than eleven (11) microns in diameter), are drained off as they fall, typically under the influence of gravity, to a vertically lower aspect located collecting surface of an aerosol chamber of the pneumatic nebulizer system. The finer droplets, however, are entered into the sample flow port of the torch, after which they are caused to transverse the vertically oriented length of the sample injector tube and eject into the region of the torch in which a plasma can be formed. Typically said motion is caused by a pressure gradient which results from the entering of a carrier gas flow along with the sample. Analysis is performed when the sample so entered is subjected to the plasma, interacts therewith, and causes identifying EM wave radiation to be emitted, or sample fragments to be produced. EM wave detecting equipment, and/or sample fragment analysis equipment positioned to intercept the emitted EM wave radiation, or to intercept sample fragments, then develops frequency or mass spectrum information respectively, which can be used to identify the elemental and molecular components of a sample. Existing torches, as described above, work relatively well when provided a sample produced by a low efficiency nebulizer. Such a sample is properly termed a "low solids" content sample. This is the case as a large portion, (e.g. typically over ninety-eight (98% ) percent), of the nebulized sample solution droplets created by a pneumatic nebulizer are of a relatively large diameter and do not enter the torch. Hence, it will be appreciated that a large portion of the solids in a sample entered into a low efficiency nebulizer, (e.g. typical pneumatic nebulizers), for nebulization is lost.
Recently, CETAC TECHNOLOGIES INC., of Omaha, Nebr. has commercialized a different type of nebulizing equipment. The descriptive term for the new nebulizing equipment is "Ultrasonic Nebulizer". Said device provides sample solution droplets which contain a far greater content of relatively small diameter droplets, (e.g. diameters less than eleven (11) microns). As a result, far more of the sample solution droplets (thirty (30) times or more as compared to the result created by a low efficiency pneumatic nebulizer), are available for entry into a torch for analysis. That is, far less of the sample solids of a sample solution are drained off in the form of relatively large droplets which never reach the torch sample flow port. This, it will be appreciated, can provide a far larger solids content nebulized sample to a torch. (This is especially the case when the solids content of a sample solution which is nebulized exceeds one-half (0.5%) to one (1%) percent thereof, thereby constituting a high solids content solution). The sample produced by the CETAC ultrasonic nebulizer is, thus, termed a "High Solids" content sample. The Ultrasonic Nebulizer, it is mentioned, provides the superior sample solids content, as compared to that provided by a pneumatic nebulizer, by using the vibrational energy which is developed by a piezoelectric crystal or equivalent, when operating as an element of an oscillating circuit. Typically, but not necessarily, such a piezoelectric crystal or equivalent and oscillator circuit will be structured so as to cause the piezoelectric crystal or equivalent to vibrate at one-and-three-tenths (1.3) Megahertz. When this frequency is used it has been found that the CETAC system provides over thirty (30) percent of sample solution droplets with a diameter of eleven (11) microns or less. In use, a solution of sample solids and a solvent are caused to impinge upon said vibrating piezoelectric crystal or equivalent, or in close proximity thereto, and thereat are instantaneously transformed into nebulized sample solution droplets. The Ultrasonic Nebulizer of CETAC TECHNOLOGIES INC. also incorporates a desolvating chamber in which the solvent is removed from the nebulized sample solution droplets, thereby providing desolvated, fine particles of sample for entry to a sample flow port of a plasma torch analysis system. By control of the desolvation process, it is noted, the solvent vapor content of the resulting high solids content nebulized sample can be controlled.
It is desirable to provide higher solids content samples to a torch, based upon less sample solids loss in the nebulizing and desolvation processes, as the sensitivity of the overall analysis equipment system is increased. However, problems have been found to exist when a typical standard torch, as described above, is used with high solids content samples. In particular, the standard typically available torches tend to be prone to retain solids from a sample introduced in one analysis procedure, and release said retained sample solids during subsequent analysis procedures. This effect is termed a "carry-over" or "memory" effect. Possibly more frustrating, however, than even the sample carrier over effect, is the torch clogging effect. That is, the entry of higher solids content samples to a sample flow port of a standard torch can lead to the standard torch actually becoming clogged by adherence to and accumulation of said sample solids on various components of the standard torch. One particularly obvious example of a component of a standard torch which is susceptable to clogging is the tapering of the sample injector tube. As discussed above, said taper is normally present to cause an increase in the ejection velocity of sample particle flow which leaves the vertically upper aspect of a sample injection tube.
A search of Patents produced no teachings which are directly on point. That is, no prior Patents were found which directly focus upon providing a torch, or torches which are specifically designed to be used with high solids content nebulized samples and which serve to minimize sample carry-over and clogging effects when used with high solids content samples. A Patent to Dorn et al. U.S. Pat. No. 4,980,057 teaches the use of an ultrasionic nebulizer in conjunction with pneumatic means to nebulize samples, and another Patent, U.S. Pat. No. 4,109,863 to Olsen et al. teaches an ultrasonic nebulizing system in which the piezoelectric or equivalent therein is secured to and encased in an enclosed, protective, hollow body. In addition, Patents which teach some interesting torch designs were found. For instance, U.S. Pat. No. 3,467,471 and U.S. Pat. No. Re. 29,304 to Greenfield et al. teach torches which provide an additional concentric tube within the sample injection tube of the standard torch. In use the additional tube carries sample and a gas jet is injected into the annular space between the outer surface of the additional tube, and the inner surface of what would be the sample injector tube were the additional tube not present. The gas jet causes a reduced pressure to develop at the vertically upper aspect of the additional tube. Said additional tube, at its vertically lower aspect, is placed into a source of sample and said sample is, by the Bernouli effect caused to flow through the additional tube. Mention is made of injecting an aerosol, (i.e. nebulized) sample into the additional tube, but such is discouraged by the Greenfield et al. teachings. The explanation given is that then known methods of preparing sample in an aerosol form causes loss of a large amount of sample. It is emphasised that this is the very problem which use of the high efficiency CETAC ultrasonic nebulizer in the present invention has overcome. As a result, certain aspects of the Greenfield et al. inventions might be found to provide a benefit, when used as described in other sections of this Disclosure, not appreciated by Greenfield et al., when said Greenfield et al. invention is considered in view of the present state of the art regarding sample nebulization techniques. In addition, Japanese Patents owned by the Shimadzu Corporation, Nos. JA 0136052, JA 0210754, JA 0052748 and JA 0143936 also teach torches with a similar geometry to that taught in the Greenfield Patents. However, if a gas is caused to flow in the space between the outer surface of the additional tube and the inner surface of what would be the sample injector tube in a standard torch, it is not caused to do so simultaneous with an aerosol form sample flow through the additional tube. One such Patent, No. JA 0136052 uses the identified gas flow as a means to allow easy extinguishing of a filament plasma. The invention in JA 0210754 uses the identified gas flow to lift a sample vapor into a plasma, which sample vapor is created near the plasma by a heating means. The invention in JA 0143936 uses said gas flow in an alternating schedule with a flow through the additional tube. Both flows include sample therein, and the torch is designed with a view to allowing easy construction of a calibration curve. A Patent to Tamaka et al. U.S. Pat. No. 4,578,560 teaches the use of concentric tubes, (termed pipes in said Patent), which can be combined to form a torch with multiple tubes present. Flanges on said tubes facilitate the combining of said tubes. Other Patents teach the use of flowing fluid to minimize overheating of a torch during use. For instance, a Patent to Rayson et al., U.S. Pat. No. 5,012,065, teaches that a gas should be directed to flow in a laminar fashion along the inner surface of the outer tube. The presence of inserts between the outer and intermediate tube provide longitudinal channels which govern said gas flow. Another Patent, U.S. Pat. No. 4,551,609 to Falk teaches that a dead space should be sealed between the lower portions of the sample injector tube (termed a capillary tube in said Patent), and the intermediate tube (termed an inner jacket in said Patent). The closing of said dead space apparently reduces the amount of cooling gas required to be entered to the equivalent of the outer port in said invention. A Patent to Seliskar et al., U.S. Pat. No. 4,794,230 also teaches an invention which provides cooling to the outer components of the torch therein. However, the cooling is provided by water which flows within an additional tubular element which surrounds the equivalent of the outer tube in said invention, over the outer surface of the equivalent of the outer tube.
A need for a torch, or torches, which are specifically designed to allow use so as to limit the tendency for sample carry over and/or torch clogging when high solids content samples are introduced thereto, is thus identified. The need has recently been rendered critical by the availability in the market of the highly efficient ultrasonic nebulizer produced and marketed by CETAC TECHNOLOGIES INC.